Crochet hook sizes and their conversions
Hooks by yarn weight, the millimetre against the US letter, the old UK number beside both, and the rule for when to go up or down half a size.
A crochet hook is named by the diameter of its shaft in millimetres, and every other label sits on top of that. The working range runs from about 0.6mm for thread to 25mm for arm work; the two hooks most patterns name are 5mm (US H/8) for worsted and 4mm (US G/6) for DK. Millimetres are exact and international. The US letter is not: one maker's J/10 is 6.0mm and another's is 6.5mm, so the letter is a hint and the number is the instruction. The rule here is short — half a millimetre down from the ball band for anything that will be stuffed, half a millimetre up for anything that has to hang, and a swatch either way before a measurement matters.
| Millimetres | US | Old UK | Yarn it suits | Used for |
|---|---|---|---|---|
| 2.25mm | B/1 | 13 | Lace, thread | Edgings, thread motifs |
| 2.75mm | C/2 | 12 | Lace, sock | Fine motifs, small lace |
| 3.25mm | D/3 | 10 | Sock, fingering | Shawls, socks |
| 3.5mm | E/4 | 9 | Sport, DK | Small creatures, baby pieces |
| 3.75mm | F/5 | 9 | DK, light worsted | DK garments and blankets |
| 4mm | G/6 | 8 | DK, light worsted | The standard DK hook |
| 4.5mm | 7 | 7 | DK, worsted | Worsted worked firm |
| 5mm | H/8 | 6 | Worsted | The standard worsted hook |
| 5.5mm | I/9 | 5 | Worsted, aran | Aran, and worsted with fall |
| 6mm | J/10 | 4 | Aran, bulky | Throws, heavy garments |
| 6.5mm | K/10½ | 3 | Bulky | Fast blankets |
| 8mm | L/11 | 0 | Super bulky | Baskets, chunky throws |
| 9mm | M/13 | 00 | Super bulky | Floor cushions |
| 10mm | N/15 | 000 | Jumbo, roving | Oversized accents |
| 15mm+ | P/Q/S | — | Jumbo | Rugs, arm work |
Straight answers, in one place
What is the right first hook to own?
5mm, US H/8, with a medium-weight yarn. The stitches are large enough to see what your hands are doing, and the yarn is thick enough to pull back and rework without fraying.
Which hook closes a fabric tightly enough to hold filling?
4mm or 4.5mm with a medium yarn — half a size to a full size under what the ball band asks for. The fabric has to close tightly enough that the filling stays behind it.
How many millimetres is a US H?
5mm. It is often printed H/8 on the handle.
Which US letter is stamped on a 4mm?
G, usually printed G/6.
Half a millimetre up or half a millimetre down — how is that decided?
Down for anything stuffed or structural, up for anything that has to hang. Then swatch, because the hand on the hook counts for more than the half millimetre does.
Do two makers' hooks of the same name measure alike?
The millimetre does. The letter does not — it drifts by up to half a millimetre between makers, which is enough to change a gauge. Buy and read by the number.
Picking the hook when a pattern does not name one
Start at the ball band. The hook printed there is the one the spinner used to reach the gauge printed beside it, so it is a measurement rather than a suggestion. Then read what the piece has to do. A fabric that will hold filling wants to be tighter than the band's, so take half a millimetre off. A fabric that has to hang — a shawl, a light garment, a blanket you want to fall over the arm of a chair rather than stand on it — wants half a millimetre on.
Then swatch. Four inches square, in the stitch the pattern uses, worked and counted. If the count comes out high, the fabric is tighter than the pattern expects and the hook goes up; if it comes out low, the hook goes down. Two swatches is not a wasted evening when the alternative is a garment that fits nobody.
Why the millimetre wins over the letter
Millimetres measure the shaft, so they mean the same thing in every country and every catalogue. The US letters are a scale that grew by habit: correctly ordered, unevenly spaced, and mapped to millimetres slightly differently by different makers before anyone settled it. J/10 is the standing example — 6.0mm from one maker, 6.5mm from another, and half a millimetre is enough to move a gauge by a stitch over four inches.
The old UK numbers run the other way, largest number to smallest hook, which is worth knowing if you are reading a printed British book from before the metric column appeared. This page carries them so a vintage instruction can be followed, not because anything is sold by them now. Where a pattern names a hook it gives the millimetre first and the letter in brackets.
What the hook is made of
Two things get chosen in the same moment here, and only one of them is the hook.
The shaft: aluminium, bamboo, steel
Aluminium is the default: slick, cheap, and the yarn travels over it without being coaxed. Bamboo and wood grip a little more, which is a help with slippery mercerised cotton and a nuisance with a matte yarn that is already reluctant to move. Steel appears only at thread sizes, under 2mm, where anything softer would flex under the pull.
The handle, which is what the hand actually meets
The part worth spending on is the handle rather than the hook. An hour of tight single crochet is an hour of the same grip, and a cushioned or flattened handle is what keeps a hand working through it. Head shape matters quietly too: an inline head splits a stitch less often, a tapered head slides into a tight stitch more easily, and most people settle on one within a few projects without ever learning the word for it.